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The limbic system is a term for a set of brain structures including the hippocampus and amygdala that support a variety of functions including emotion, behavior and long term memory. The structures of the brain described by the limbic system are closely associated with the olfactory structures.Medline Plus Medical Encylcopedia[http://www.nlm.nih.gov/medlineplus/ency/imagepages/19244.htm] The term "limbic" comes from Latin limbus, meaning "border" or "edge".
Anatomy The limbic system includes many structures in the cerebral cortex and sub-cortex of the brain. The term has been used within Psychiatry and Neurology, although its exact role and definition has been revised considerably since the term was introduced.Conn, Michael P. 2003. Neuroscience in Medicine, 370 The following structures are, or have been considered to be part of the limbic system:
Function The limbic system operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens, the brain's pleasure center, which plays a role in sexual arousal and the "high" derived from certain recreational drugs. These responses are heavily modulated by dopaminergic projections from the limbic system. In 1954, Olds and Milner found that rats with metal electrodes implanted into their nucleus accumbens repeatedly pressed a lever activating this region, and did so in preference to eating and drinking, eventually dying of exhaustion.Olds, J., Milner, P. 1954. Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. J.Comp. Physiolo. Psycholo. 47, 419–427 The limbic system is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. To cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery, called a prefrontal lobotomy (this is actually a misnomer). Patients who underwent this procedure often became passive and lacked all motivation.There is circumstantial evidence that the limbic system also provides a custodial function for the maintenance of a healthy conscious state of mind.
Evolution The limbic system is embryologically older than other parts of the brain. It developed to manage 'fight' or 'flight' chemicals and is an evolutionary necessity for reptiles as well as humans.Recent studies of the limbic system of tetrapods have challenged some long-held tenets of forebrain evolution. The common ancestors of reptiles and mammals had a well-developed limbic system in which the basic subdivisions and connections of the amygdalar nuclei were established.
History The French physician Paul Broca first called this part of the brain "le grand lobe limbique" in 1878,Broca, P. Anatomie comparée des circonvolutions cérébrales: le grand lobe limbique. Rev. Anthropol. 1878;1:385–498. but most of its putative role in emotion was developed only in 1937 when the American physician James Papez described his anatomical model of emotion, the Papez circuit.Papez JW. A proposed mechanism of emotion. 1937. J Neuropsychiatry Clin Neurosci. 1995;7(1):103-12. PMID 7711480 Paul D. MacLean expanded these ideas to include additional structures in a more dispersed "limbic system," more on the lines of the system described above.Maclean, PD. Some psychiatric implications of physiological studies on frontotemporal portion of limbic system (visceral brain). Electroencephalogr Clin Neurophysiol 1952;4(4):407-18. PMID 12998590 The concept of the limbic system has since been further expanded and developed by Nauta, Heimer and others.
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